A contact lens (also known simply as a contact) is a corrective, cosmetic, or therapeutic lens usually placed on the cornea of the eye. Contact lens usually serve the same corrective purposes as glasses, but are lightweight and virtually invisible. Some cosmetic lenses are deliberately colored to alter the appearance of the eye. However, the majority of contact lens users rely on the contact lens for vision correction to enable them to perform day to day tasks without the need for glasses.
Modern soft contact lenses were invented by the Czech chemist Otto Wichterle and his assistant Drahoslav Lím, who also invented the first gel used for their production. The first contact lenses were made of glass, which caused eye irritation, and were not wearable for extended periods of time. But when William Feinbloom introduced lenses made from polymethyl methacrylate (PMMA or Perspex/Plexiglas), contacts became much more convenient. These PMMA lenses are commonly referred to as “hard” lenses.
However, PMMA lenses have their own side effects: no oxygen is transmitted through the lens to the cornea, which can cause a number of adverse clinical events. In the late 1970s, and through the 1980s and 1990s, improved rigid materials—which were also oxygen-permeable—were developed. Collectively, these polymers are referred to as rigid gas permeable or ‘RGP’ materials or lenses. One advantage of hard lenses is that, due to their non-porous nature, they do not absorb chemicals or fumes. The absorption of such compounds by other types of contacts can be a problem for those who are routinely exposed to painting or other chemical processes.
Rigid lenses offer a number of unique properties. In effect, the lens is able to replace the natural shape of the cornea with a new refracting surface. This means that a regular (spherical) rigid contact lens can provide good level of vision in people who have astigmatism or distorted corneal shapes as with keratoconus.
While rigid lenses have been around for about 120 years, soft lenses are a much more recent development. The principal breakthrough in soft lenses made by Otto Wichterle led to the launch of the first soft (hydrogel) lenses in some countries in the 1960s and the approval of the ‘Soflens’ material (polymacon) by the United States FDA in 1971. Soft lenses are immediately comfortable, while rigid lenses require a period of adaptation before full comfort is achieved. The polymers from which soft lenses are manufactured improved over the next 25 years, primarily in terms of increasing the oxygen permeability by varying the ingredients making up the polymers.
A small number of hybrid rigid/soft lenses exist. An alternative technique is piggybacking of contact lenses, a smaller, rigid lens being mounted atop a larger, soft lens. This is done for a variety of clinical situations where a single lens will not provide the optical power, fitting characteristics, or comfort required.
In 1999, ‘silicone hydrogels’ became available. Silicone hydrogels have both the extremely high oxygen permeability of silicone and the comfort and clinical performance of the conventional hydrogels. These lenses were initially advocated primarily for extended (overnight) wear, although more recently daily (no overnight) wear silicone hydrogels have been approved and launched.
While it provides the oxygen permeability, the silicone also makes the lens surface highly hydrophobic and less “wettable.” This frequently results in discomfort and dryness during lens wear. In order to compensate for the hydrophobicity, hydrogels are added (hence the name “silicone hydrogels”) to make the lenses more hydrophilic. However, the lens surface may still remain hydrophobic. Hence some of the lenses undergo surface modification processes which cover the hydrophobic sites of silicone. Some other lens types incorporate internal rewetting agents to make the lens surface hydrophilic.
Cosmetic contact lenses are designed to change the appearance of the eye. These lenses may also correct the vision, but some blurring or obstruction of vision may occur as a result of the color or design. In the USA, the Food and Drug Administration frequently calls non-corrective cosmetic contact lenses decorative contact lenses. These types of lenses tend to cause mild irritation on insertion, but after the eyes become accustomed, tend to cause no long term damage. Though it is advised that these lenses not be worn too much, research has shown them to have no direct link to any forms of eye degradation.
Although it is preferable that none cosmetic contact lenses be perfectly clear, many brands of contact lenses are lightly tinted to make them easier to handle. Many commercial lenses are tinted a faint blue to make them more visible when immersed in cleaning and storage solutions. The problem of handling a contact lens is compounded in the event the contact lens is not in a know location, such as in the situation when the lens falls out of a person's hand or eye. The difficulty of finding a lens which has fallen is well known by all contact lens users.
Common problems experienced by regular contact lens wearers, whether they use clear lenses or a tinted lenses, are: (1) retrieving a contact lens from the solution within which it is stored, (2) distinguishing between the right and left lenses which may lead reverse installation, and (3) locating a lens in the event it falls out of the eye or off the finger during insertion or removal. Although problems 1 and 3 may be more obvious, problem 2 is no less important. For users who have different prescriptions between the left and right eye, accidently inserting a contact with a prescription for the right eye into the left eye, and/or visa versa, may lead to impaired vision, discomfort and headaches. There is, therefore, a need in the field of ophthalmics for improved contact lenses and methods of producing same.